Sunrise, Sunset, & Star-rise, Star-set

The position of the sunrise shifts along the horizon over
the course of a year. The position of sunset also shifts by the same
amount. How
far this migration of the sunrise or sunset extends along the horizon
depends on where you live. At the equator, the sunrise position migrates
plus or minus 23.5° from due East, while at Minneapolis, which is
located 45°N latitude, the position can vary about plus or minus
35° from due East. At the arctic circle the change is plus or minus
90° which is the maximum possible amount.

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Ancient astronomers could easily mark the extreme positions
of sunrise and sunset. All they had to do was stand in the same spot
every day (a fixed observing position) and use large stones, trees,
or other landmarks to mark the point in their view where the sun rose
or set. These observations of the sunrise position could then be used
to set up a calendar for the year.

In contrast, the rising and setting positions of stars
don't change over the course of a year, but the rising and setting times
do change. Using a fairly accurate clock you could observe that a given
star sets (and rises) 4 minutes earlier per day. The average digital
wristwatch more than qualifies to use for measurements in this case.

If you add this time up over the course of a month is
comes to about 2 hours earlier per month, or 24 hours per year. So if
a star rises at 3AM today, what time will it rise one year from today?
Answer: It will rise at the same time on the same date every year (give
or take a day depending on where we are in the leap year cycle).

A result of this gradual change in rising and setting
times is that different constellations are up in the evening at different
times of the year. Orion, Taurus, Gemini, and Canis Major and Minor
are high in the sky to the South in the winter evenings as seen from
North America or Europe. In the summer evenings, Scorpius and Sagittarius
are most obvious in the southern sky. The "summer triangle"
is very high in the sky for these same locations during June, July and
August evenings.

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Another ancient calendar marker was the first time that
a certain star could be seen in the glow of morning twilight, rising
ahead of the Sun. This phenomenon was called "heliacal rising".

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First, the Sun rises ahead of the star. Then, the star
is lost in the glow of sunrise. Finally, the star is visible ahead of
the sunrise, and we have our marker for the calendar.

The moon and the planets, on the other hand, have more
complicated apparent motions in the sky. As you may have noticed in
Activity One, Mars scoots around the sky fairly quickly, passing through
one constellation of the zodiac every two months, and showing up in
the evening sky about once every two years. In contrast, Neptune and
Pluto, move very slowly with respect to the stars. Pluto, for instance,
shifts by less than 2° per year against the background stars. This
makes sense, because Mars is much closer than Pluto and the motion of
the closer planet is easier to detect. In addition, planets farther
from the Sun move more slowly. Pluto, for example, goes about 40 times
as far in its trip around the Sun as we do. It takes Pluto 250 years
to travel once around, so its average speed is 40/250 or 16% of Earth's
speed around the Sun.

The Moons motion changes the fastest of any body
we can see. Once about every 29 days the Moon appears as a thin crescent
in the evening sky just after Sunset and sets shortly after the Sun
sets. Later in the month, the moon appears to slowly grow fatter and
sets later and later in the evening. Eventually, the moon rises at sunset
and is full in the sky all night. The moon again rises later and later,
the lighted portion becomes thinner. Finally, the moon is a thin crescent
rising just before the Sun rises. Soon after that, the cycles starts
over again.

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If
you observe Venus in the evening sky, you may notice that it appears
higher and higher at sunset for a while. Then, it will reverse direction
and be lower and lower until you can't see it any more in the glare
of sunset. If you then start watching for it in the morning sky, you'll
see it higher and higher at sunrise, until it again reverses. This whole
cycle, the synodic period of Venus, takes about 584 days or 1.6
years. Mercury has a similar pattern, although it is harder to see because
it never gets very far from the Sun. Mercury also has a shorter synodic
period. The other planets can appear any distance from the Sun along
the ecliptic, but Venus and Mercury stay close to the Sun. Why?